Method and apparatus for the diagnosis of sleep apnea utilizing a single interface with a human body part

ABSTRACT

A device for diagnosing sleep apnea by identifying desaturation and resaturation events in oxygen saturation of a patient&#39;s blood. The slope of the events is determined and compared against various information to determine sleep apnea.

This is a continuation-in-part of PCT/US93/07726 filed Aug. 19, 1993 andof Application Ser. No. 07/931,976 filed Aug. 19, 1992, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

Disorders of breathing during sleep are now known to constitute a majorhealth problem throughout the world. Obstructive sleep apnea is anextremely common disease which manifests itself in variable degrees ofseverity. The disease develops when muscle tone of the upper airwaydiminishes during sleep and negative pressures associated withinspiration result in collapse of the upper airway, preventing airmovement and resulting in airway obstruction. The sleeping patientinhales more forcibly, thereby, further lowering upper airway pressuresand causing further collapse of the upper airway. During this time,substantially no air movement into the chest occurs and the patientbecomes progressively more hypoxic and hypercarbic. Both hypoxemia andhypercarbia produce central nervous system stimulation resulting inarousal. Upon arousal, increase in airway muscle tone opens the airwayand the patient rapidly inhales and ventilates quickly to correct theabnormal arterial blood gas values. Generally, the arousal is onlymodest and the patient is not aware of the arousal. Once blood gasparameters have been corrected, the patient begins to sleep more deeply,upper airway tone again diminishes, and the upper airway collapsesresulting in sequential and cyclic apneic arousal episodes.

The duration and severity of each apnea is quite variable from patientto patient and with the same patient throughout the night. Indeed, thedisease process represents a spectrum of severity from mild snoring,which is associated with incomplete and inconsequential airwayobstruction, to severe apneas which can result in fatal hypoxemia.

This disease commonly results in excessive daytime sleepiness and candisrupt cognitive function during the day due to fragmentation of sleepduring the night associated with recurrent arousals of which the patientis not aware.

Although this disease commonly affects obese patients, it may occur inpatients with any body habitus. Because this disease is so common andbecause it presents with the subtle and common symptoms of excessivedaytime sleepiness, morning headache, and decreasing ability toconcentrate during the day, it is critical that an inexpensive techniquefor accurately diagnosing and treating this disease be developed.Traditionally, this disease has been diagnosed utilizing a complex andexpensive multi-channel polysomnogram. This is generally performed in asleep lab and involves the continuous and simultaneous measurement andrecording of an encephalogram, electromyogram, extraoculogram, chestwall plethysmogram, electrocardiogram, measurements of nasal and oralair flow, and pulse oximetry. These, and often other, channels aremeasured simultaneously throughout the night and these complexrecordings are then analyzed to determine the presence or absence ofsleep apnea.

The problem with this traditional approach is that such complex sleeptesting costs between one-thousand to thirty five hundred dollars. Sincesleep apnea is so common, the cost of diagnosing obstructive sleep apneain every patient having this disease in the United States would exceedTen Billion Dollars. It is critical that a new, inexpensive technique ofaccurately diagnosing sleep apnea be developed.

Nocturnal oximetry alone has been used as a screening tool to screenpatients with symptoms suggestive of sleep apnea to identify whether ornot oxygen desaturations of hemoglobin occur. Microprocessors have beenused to summarize nocturnal oximetry recordings and to calculate thepercentage of time spent below certain values of oxygen saturationHowever, oxygen desaturation of hemoglobin can be caused by artifact,hypoventilation, ventilation perfusion mismatching. For these reasons,such desaturations identified on nocturnal oximetry are not specific forsleep apnea and the diagnosis of sleep apnea has generally requiredexpensive formal polysomnography.

The present invention comprises a system and technique for deriving andutilizing the analysis of graphical pulse oximetry-derived waveforms asa function of time to accurately diagnosis sleep apnea with adequatespecificity to, in many cases, eliminate the need for expensive formalpolysomnography.

It is the purpose of this invention to provide an inexpensive system forthe collection and analysis of pulse oximetry values as a function oftime during sleep to provide a diagnosis of sleep apnea with a highdegree of specificity.

This invention provides a reliable and specific means for the diagnosisof obstructive sleep apnea which can be performed in the patient's homewithout attendance of technical personnel. It is further the purpose ofthis invention to provide an inexpensive and accurate means to bothscreen for and specifically diagnose obstructive sleep apnea by a singleovernight recording in the patient's home without the need for multipleconnections to different parts of the patient's body. It is further thepurpose of this invention to define a technique for diagnosingobstructive sleep apnea utilizing the calculation of the ascending anddescending slope ratio of phasic oxygen desaturations measured duringsleep.

Specifically, the present invention defines a device for diagnosingsleep apnea, that has the following components. First, a means mustdetermine an oxygen saturation of a patient's blood. This saturationvalue is coupled to a means for identifying a desaturation event basedon the saturation value. The desaturation event is one in which saidoxygen saturation falls below a baseline level by a predetermined amountand for a predetermined time. The slope of the event is calculated bymeans for calculating a slope of said desaturation event representing arate of change per unit time of fall of oxygen saturation. This slope isused by a means for comparing said calculated slope with a value ofslope which is determined in advance to be indicative of sleep apnea,and determination of diagnosis of sleep apnea is made based on saidcomparing.

The comparing can be done by:

1) comparing with an absolute number which is likely to indicate a sleepapnea, or

2) comparing with other slopes taken at different times.

The identifying means can also identify a resaturation, immediatelyfollowing said desaturation and coupled with said desaturation, in whichthe oxygen saturation rises, and wherein the determination can also bebased on a slope of said resaturation.

Many other ways of calculating the slope are also disclosed herein.

These and other aspects of the invention will now be described in detailwith reference to the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the basic system of the presentinvention;

FIG. 2 shows a basic flowchart of operation of the present invention;

FIGS. 3 and 4 show basic desaturation events and many of the parametersassociated therewith;

FIG. 5 shows a specific way in which a comparison can utilize thecalculation of the area above each desaturation event compared to areaabove each coupled resaturation event.

FIG. 6 shows slope of oxygen saturation as a function of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventor of the present invention found, relative to sleep apneadiagnosis, that the waveform pattern of oximetry during a sleeprecording can be considered in relation to the physiologic parametersWhich affect oxygen saturation over time. Specifically, during an apneicperiod, arterial oxygen saturation initially falls as a function of theoxygen saturation of mixed venous blood and oxygen uptake from residualexchangeable oxygen within the lungs. Subsequently, arterial oxygensaturation falls directly as a function of oxygen consumption and globaloxygen stores. These stores of oxygen are very limited. The sources ofoxygen available during an apneic period include residual exchangeableoxygen within alveoli and airways, the oxygen bound to hemoglobin,dissolved oxygen within body tissues and oxygen stored as myoglobin.These stores are rapidly depleted during an apneic period as a functionof global oxygen consumption. As oxygen stores are depleted, thecellular oxygen levels fall, and mixed venous oxygen saturationprogressively diminishes. Since a small amount of exchangeable oxygensupply exists within alveoli and airways, arterial oxygen saturation, asmeasured by the pulse oximeter is briefly unaffected by the initial fallin body oxygen storage. However, since oxygen stores within the alveoliare extremely limited, arterial oxygen saturation then progressivelyfalls toward that of mixed venous arterial blood saturation since littlesignificant gas exchange occurs as mixed venous blood passes byessentially unventilated alveoli. The partial pressure of oxygen inarterial blood therefore progressively falls toward the mean partialpressure of oxygen in body tissues at the cellular level.

It is possible to measure indirectly the partial pressure of oxygen inarterial blood by measurement of arterial oxygen saturation ofhemoglobin utilizing a pulse oximeter 12. If the probe 13 of pulseoximeter is placed on a patient's finger or other body part during aprolonged apneic period, a progressive decrement in arterial saturationwill be identified as a function of the fall in arterial oxygen partialpressure. Although the initial decline in arterial oxygen saturation isgreatly dependant on mixed venous oxygen saturation, since body oxygenstores during a apnea cannot be repleted, the subsequent portion of thefall in arterial oxygen saturation as measured by a pulse oximeter overtime will be directly correlated to the oxygen consumption of thepatient. The average oxygen consumption of a resting human (approx. 3.5ml/kg/min) has a relatively constant relationship to average bodyarterial oxygen stores (approx. 25 ml/kg). Although substantialvariability exists in body oxygen stores in chronically ill patientswith low cardiac output states (resulting in lower mixed venous oxygenstorage), a finite range of oxygen stores exists. Indeed, even in thepresence of severe compensated disease, mixed venous oxygen saturationgenerally ranges from 50-80%. Therefore, a sleeping human has adefinable and predictable range of slopes of arterial oxygen saturationdecrement as a function of the baseline mixed venous oxygen saturationinitially and of oxygen consumption and body oxygen stores terminally.Although augmented body muscular activity associated with obstructiveapnea could modestly increase oxygen consumption and although a decreasein oxygen consumption may occur below a critical levels of tissueoxygenation, the declining range of slope of desaturation is stillpredictable within only modest variances.

To understand the predictable parameters of arterial pulse oximetrywaveform, it is important to consider the way in which pulse oximetryreflects total body oxygen stores. Total body oxygen stores can beconceived as representing four major compartments:

1. The Lung Compartment,

2. The Arterial Compartment,

3. The Tissue Compartment, and

4. The Venous Compartment.

Oxygen enters the lungs and is stored sequentially in each of thesecompartments. When oxygen is depleted during apnea, depletion occursfirst in the tissue compartment, second in the venous compartment, thirdin the lung compartment, and fourth in the arterial compartment.Whereas, when oxygen is repleted, oxygen appears first in the lungcompartment, second in the arterial compartment, third in the lungcompartment, and fourth in the venous compartment. It can be seen,therefore, that since pulse oximetry measurements reflect oxygen storedwithin the arterial compartment, if sequential depletion of arterialsaturation occurred due to phasic apneas that the initial apneic episodewould result in depletion of the arterial compartment only after thesubstantial depletion of other compartments has developed.

Using the above, the inventor of the present invention realized that hecould predict with reasonable certainty whether or not a desaturationoccurring during a continuous nocturnal oximetry measurement fallswithin the anticipated range of parameters which define the slope ofarterial oxygen desaturation of hemoglobin which can physiologicallyoccur during an apneic episode. In this manner, each desaturationepisode can be defined, as a function of the characteristics of thewaveform of deflection, as either consistent with an apneic episode orinconsistent with an apneic episode. Saturations which decrease toorapidly to be accounted for on the basis of physiologic oxygen depletiondue to apnea would be identified as inconsistent with an apneic episodeand therefore identified as being secondary to artifact. On the otherhand, the desaturation episodes which decrease too slowly to beaccounted for on the basis of physiologic oxygen depletion and would beidentified as inconsistent with an apneic episode and thereforesecondary to either hypoventilation, alterations in ventilationperfusion matching, or to artifact. The means for identifying adesaturation event is preferably a processor; and according to the firstembodiment of this invention, as described above, the .processorcompares a calculated slope of the event with a value of slope which isdetermined in advance to be indicative of sleep apnea. A diagnosis ofsleep apnea is made based on that comparison.

More specifically, the preferred embodiment of the sleep apnea diagnosissystem 10 of the present invention is shown in FIG. 1. It includes aconventional pulse oximeter (12) with a probe (14) for transilluminationor reflection from a human body part such as a finger (16). The oximeteris connected to a microprocessor (20) which records oxygen saturationand pulse as a function of time. A printer (24) is connected to themicroprocessor. The microprocessor analyzes the oxygen saturation valuesas a function of time, as will be discussed in detail herein. In onepreferred embodiment, the system is used in the following way:

The microprocessor is disposed in connection with the oximeter with aprobe and printer for recording the oxygen saturation as a function oftime, and the oximeter probe is attached to a patient. The oxygensaturation of hemoglobin is recorded as a function of time while thepatient sleeps.

A measurement interval of, for example, 10 minutes is defined along thesleep recording as shown in step 200 of FIG. 2. Step 202 defines a meanmaximum baseline range of oxygen saturation of hemoglobin (±3%saturation) is defined over the measurement interval.

A desaturation event can be defined as at least a 4% substantiallyuninterrupted decrement in saturation below the defined baseline mean ofoxygen saturation. A lower percentage can be used to increasesensitivity. Each desaturation event is identified in step 204, and thedesaturation change of each desaturation event is measured. Thedesaturation interval is defined as the duration of the uninterrupteddecline in saturation of each desaturation event.

Then, slopes are calculated. The descending slope of each desaturationevent is calculated as:

    ΔS.sub.D /ΔT.sub.D

where:

ΔS_(D) =Desaturation change (in % saturation;

ΔT_(D) =Desaturation interval (in seconds).

A resaturation event is defined as a substantially uninterrupted rise insaturation which terminates the declining slope of the desaturationevent. The resaturation change of each resaturation event is alsomeasured.

The resaturation interval is measured as the duration of theuninterrupted rise in saturation of each resaturation event. Theascending slope of each resaturation event is calculated as:

    ΔS.sub.R /ΔT.sub.R.

where:

ΔS_(R) =Resaturation change (in % saturation );

ΔT_(R) =Resaturation interval in seconds.

A phasic desaturation event is defined using all coupled desaturationand resaturation events wherein the sum of the duration of thedesaturation event and the resaturation event is less than 3.5 minutesand wherein the descending slope falls within a finite range of between1.3%/sec and 0.3%/sec.

The descending to ascending saturation slope ratio of each phasicdesaturation event is calculated as:

    (ΔS.sub.D /ΔT.sub.D)/(ΔS.sub.R /ΔT.sub.R).

The number of probable apneic events within the measurement interval isdefined as the number of phasic desaturation events falling within thefinite range of ascending to descending slope ratios of between3.5-10.5.

Each probable apneic event is marked with the identity marker, PA, andthe above steps are repeated for each additional 10 min. interval alongthe recording for the entire sleep recording.

Then, appropriate action is taken: either the pulse oximetry waveform isprinted as a function of time with each probable apneic event marked PAfor identification, or treatment of sleep apnea is either manually orautomatically administered.

The probability that a patient has sleep apnea will be a direct functionof the number of phasic desaturations which meet the above criteria forsleep apnea per hour of recording and this probability can be calculatedand printed.

Therefore, in the preferred embodiment, each desaturation event isidentified as to whether or not it meets the criteria for physiologicapnea. The number of events per hour are then calculated and thenprinted. Each desaturation event which has been identified by themicroprocessor as consistent with a physiologic apnea is so marked (suchas PA for probable apnea or PCA for physiologically consistent withapnea). The pulse oximetry waveform in the preferred embodiment is thenprinted to provide a hard copy. This printed hard copy includesidentification of each desaturation event which has been determined bythe microprocessor as consistent with a physiologic apnea. In addition,the presence of desaturation slope acceleration, as will be discussed,by comparing closely spaced consecutive desaturation slopes as in FIG. 4and such identification also provided on the printed hard copy.

This invention therefore provides a compact, single device which iseasily suitable for home use and can be simply taken home by the patientand interfaced with a body part, such as a finger, to provide bothscreening and a mechanism to provide a specific diagnosis of sleep apneawith a single overnight recording. The hard printed copy providesgraphical data which can be overread by the physician since the computerspecifically identifies the desaturation events which have beeninterpreted as consistent with sleep apnea. This provides the physicianwith the opportunity to determine whether he or she agrees with thediagnostic interpretation of the microprocessor.

The diagnosis can be treated by repeating the sleep recording duringnasal CPAP (Continuous Positive Airway Pressure) therapy. Theidentification of multiple desaturations with patterns as defined abovewhich are consistent with the physiology of apnea and which areeliminated by nasal CPAP therapy is diagnostic of apnea and furtherestablishes the parameters defining effective treatment requirements.

The invention includes the system taking additional action based on theidentification of the diagnosis of sleep apnea based on the above slopecomparison. The action can include, as in FIG. 1, the microprocessoractivating a range of nasal continuous airway pressures through apressure controller within defined limits to automatically andeffectively treat a patient's sleep apnea once the diagnosis of sleepapnea has been made by the microprocessor. Activation of flow isinitiated by the microprocessor on identification-of multiple sleepapnea-related desaturations meeting the criteria as described above. Thepressure can be titrated upward by, for example 1-2 cm H₂ O pressureincrements by the microprocessor upon identification of multipleconsecutive desaturations which are not effectively eliminated by thestarting pressure.

In this way, the invention greatly enhances the diagnostic sensitivityand specificity of nocturnal oximetry in the diagnosis of sleep apneaand to further utilize the identification of oximetry-deriveddesaturation events to trigger the storage and/or collection ofadditional sensory data concerning each desaturation event and;furthermore, the system can be utilized to automatically initiate andadjust therapy to mitigate further after following desaturation events.

In addition to a definable descending desaturation slope, oximetrymeasurements during apnea periods have other definable and predictableparameters. Importantly, apneic episodes have a definable addpredictable range of duration. It is clear that brief apneic episodes,for example with brief breath holding does not result in significantarterial oxygen desaturation as measured by pulse oximetry. However,when apneic periods are prolonged as with obstructive sleep apnea,oxygen desaturation progressively declines as a function of factors, aspreviously discussed. Unless such an apneic episode is limited induration, the patient would die from hypoxemia. Therefore, eachdesaturation which occurs as a function of apnea will have a phasicquality with a predictable range of duration. A second aspect of theinvention analyzes the duration of the apneic episode to determine if itis of a duration likely to indicate sleep apnea.

The range of duration generally does not exceed three minutes.Therefore, for a desaturation event identified by pulse oximetry to besecondary to an apneic episode, it should preferably have a duration ofless than three minutes. Oxygen desaturations due to sleep apnea shouldbe terminated with the resaturation of recovery within 3-3.5 minutes orless. Oxygen desaturation events which occur for greater than threeminutes are identified as either secondary to hypoventilation,ventilation perfusion mismatching, or artifact.

Another aspect of the invention is based on the recognition that anapneic episode which occurs during sleep is generally reversed by anarousal. At this point, the patient's central nervous system increasesupper airway tone and atmospheric gas rapidly enters the lungs andexchanges with the oxygen depleted gas within the alveoli. This exchangeoccurs within a few seconds. Since mixed venous blood in pulmonarycapillaries rapidly equilibrates with the partial pressure of oxygen inthe alveoli, arterial oxygenation will recover within seconds of therepletion of oxygen within alveoli. It should be noted that the amountof time required for blood to pass from the pulmonary capillaries to theperipheral site of pulse oximetry measurement can be measured is verybrief. Therefore, the ascending slope of oxygen saturation duringrecovery from an apneic episode is extremely rapid. Ascending slopeswhich are not rapid are unlikely to be secondary to repletion of oxygenpartial pressure within alveoli associated with arousal from an apneicepisode and rather may be secondary to a crescendo of increasingrespirations following a hypoventilation episode as in Cheyne-Stokesrespirations or may be secondary to improvement in ventilation perfusionmatching. In a recent study performed by the present inventor the meanslope of desaturation was 0.8% per second, with all desaturation slopesranging between 0.3% per second and 1.1% per second. The mean slope ofrecovery 7.6% per second, with recovery slopes ranging from 2.5% persecond to 8.3% per second. The mean recovery to apnea slope ratio was7.66, with a range of 3.8 to 10.4. Hence, in yet another aspect of theinvention, the resaturation slope, immediately following thedesaturation, is also determined, and used in the diagnosis of sleepapnea.

Additional ways of comparing the calculated slope with a value of slopewhich is determined in advance to be indicative of sleep apnea includeusing other parameters to enhance the specificity ofcontinuous-nocturnal oximetry in the diagnosis of sleep apnea includecomparisons of consecutive desaturation slope values and theidentification of alterations in desaturation values as a function ofevents occurring immediately prior to the desaturation event.

Since obstructive sleep apnea events occur by similar physiologicprocess each time within the same patient, consecutive desaturationevents will commonly have similar desaturation slopes. Theidentification of these consecutive desaturation events having similardesaturation slopes which have values consistent with physiologic apneaprovides additional evidence supporting these events as secondary tocyclic obstructive sleep apnea.

Furthermore, the preceding desaturation event can effect the shape andthe slope of the desaturation event which immediately follows. That is,preceding desaturation event may accelerate the initial portion of theslope of the following desaturation. Although other factors maycontribute to the development of this increase in desaturation slope,the primary factor appears to be the depletion of body oxygen storeswhere insufficient time has developed for repletion for tissue andvenous oxygen stores. In other words, during rapidly cycling apneicevents, recovery time may be inadequate to replete all body oxygenstores. However, the pulse oximeter is measuring arteria oxygensaturation. Therefore, after repletion of oxygen stores within the lung,arterial oxygen saturation rapidly rises before venous oxygen storeshave been repleted. If an apneic event recurs before the restoration ofvenous oxygen stores, this apneic event will be superimposed uponsubstantially depleted total body oxygen stores despite the fact thatpulse oximetry may demonstrate normal arterial oxygen saturation. Sinceat this time apnea is occurring in the presence of markedly depletedbody oxygen stores (i.e. a much lower mixed venous oxygen saturation),the initial portion of the slope of the declining arterial oxygensaturation may be substantially greater than the slope of the decline ofoxygen saturation which occurred during the preceding desaturationevent. This phenomenon would not be expected to occur in associationwith artifact and would only be expected to occur in the presence ofrapidly cycling changes in body tissue oxygen stores. Consecutiveclosely spaced desaturation events, therefore, interact so that thefirst desaturation event can affect the waveform of the seconddesaturation event provided the interval between the two events is shortenough and the level of desaturation occurring in the first event issubstantial enough to result in a sizable depletion of total body oxygenstores.

The greatest portion of oxygen storage is within the venous compartment.At any given time, therefore, the amount of global oxygen stored is, inlarge part, a function of the extent of excess of oxygen delivered tothe tissues which is stored within the venous pool. In the absence ofarterial hypoxemia or profoundly compromised cardiovascular function,oxygen delivery substantially exceeds oxygen demand, resulting inconsiderable oxygen stores within the mixed venous pool. The amount ofoxygen stored within the mixed venous pool can, therefore, be seen as adynamically-stored, hidden buffer which mitigates the decline insaturation attendant any change in alveolar ventilation. Althoughpatients with profoundly decreased mixed venous oxygen saturations wouldbe expected to have a more rapid and greater fall in arterial oxygensaturation for any given level of change in alveolar ventilation, thisstill falls within a definable range.

During very rapidly cycling apneas (i.e. apneas occurring within lessthan 10-20 seconds of each other), body oxygen stores can be seentherefore as a moving wave through consecutive body compartments whereinthe first wave affects the configuration of the second wave. Theidentification of this effect should be virtually diagnostic of rapidlycycling sleep apnea and this phenomenon can be exploited to assist inthe specific diagnosis of sleep apnea utilizing the recording ofnocturnal oximetry alone.

Desaturation slope acceleration may occur when cyclic apneic eventsoccur within less than 10 seconds of each other and when the depth ofarterial saturation associated with the first cyclic event is greaterthan 15%. The inter-desaturation event intervals can be definedspecifically as that point wherein the first desaturation event recoverssubstantially to baseline to the point in time when the seconddesaturation event begins to decline from the baseline.

It can be seen, therefore, that a declining waveform of arterial oxygendesaturation in severe sleep apnea can be expected to have two majorphysiologically-derived components: 1) the slope of the initialdeclining limb which is primarily a function of the level of mixedvenous oxygen saturation at the onset of apnea and the amount ofexchangeable oxygen in the lung remaining after the onset of apnea. 2)the second component or terminal limb is primarily a function of globaloxygen consumption relative to body oxygen stores. (The terminal limbmay not be present if apnea is brief.) The slope of the initial andterminal limb are generally similar in patients with normal mixed venousoxygen saturations. However, in patients with significantly low mixedvenous oxygen saturation, the initial limb may have a much greater slopethan the terminal limb, producing an angled appearance suggestingantecedent depletion of mixed venous oxygen stores.

The magnitude of the oxygen deficit which is derived from the precedingapneic event less the intervening excess oxygen uptake which attenuatesthis deficit between the apneas defines the magnitude of the slopeacceleration of the initial limb of the after-following desaturationevent. Therefore, an interval of oxygen deficit is present following asustained apnea but it is hidden since arterial oxygen saturation isnormal.

FIG. 3 illustrates a desaturation event and many of the parameters asdiscussed supra which define the event. The parameters shown include:

ΔS_(D) Fall in saturation (in % sat.)

ΔS_(R) Rise in saturation (in % sat.)

ΔT_(D) Duration of the fall in Saturation/desaturation (in seconds)

ΔT_(R) Duration of the rise in saturation/desaturation (in seconds)

M_(D) =ΔS_(D) /ΔT_(D) =Mean Slope of Desaturation

M_(R) =ΔS_(R) /ΔT_(R) =Mean Slope of Resaturation.

We also define the following terms:

AI The apnea interval--(the actual time wherein the patient experiencescessation of airflow which precipitates oxygen desaturation.)

OAI The occult apnea interval--(the interval wherein apnea has occurred;however, arterial oxygen stores are maintained by a shift of oxygenstores form the lung and venous compartment into the arterialcompartment hiding the fall in body oxygen stores with respect to theoximetry measurement.)

OODI The occult oxygen deficit interval--(the interval immediatelyfollowing return of oxygen saturation to near baseline after adesaturation event and wherein mixed venous oxygen desaturationpersists. If a second apnea occurs within this interval, the slope ofdesaturation may be increased).

Using these parameters and realizations discussed supra, the inventor ofthe present invention made a system and technique which automaticallyanalyzed the waveform pattern of continuous nocturnal oximetry, tospecifically identify the presence or absence of moderate to severeobstructive sleep apnea-induced arterial oxygen desaturation. Such asystem and technique makes it possible to diagnose moderate to severeobstructive sleep apnea with confidence with a single channel recordingof nocturnal oximetry alone avoiding the need for complex and expensivepolysomnography in the diagnosis of this disorder. The system andtechnique includes a mechanism to achieve the measurement of acompendium of parameters which are repetitively measured and analyzed,each improving the specificity of the diagnosis.

A summary of one such technique is as follows:

1. Dispose a microprocessor in connection with the oximeter with a probeand printer for recording the oxygen saturation of hemoglobin as afunction of time.

2. Attach the oximeter probe to a patient.

3. Define a measurement interval.

4. Define the mean maximum baseline range of oxygen saturation ofhemoglobin over the measurement interval.

5. Define-a desaturation event as at a specific uninterrupted decrementin saturation below the defined baseline range of oxygen saturation.

6. Measure the duration of the uninterrupted decline in saturation ofeach desaturation event.

7. Calculate the descending slope of each desaturation event.

8. Define a resaturation event as an uninterrupted rise in saturationwhich terminates the declining slope of the desaturation event.

9. Calculate the ascending slope of each resaturation event.

10. Define a phasic desaturation event as all coupled desaturation andresaturation events wherein the sum of the duration of the desaturationevent and the resaturation event is less than a specified value andwherein the descending slope falls within a finite range.

11. Calculate the descending to ascending saturation slope ratio of eachphasic desaturation event.

12. Define the number of probable apneic events within the measurementinterval by comparing said calculated slope with a value of slope whichis determined in advance to be indicative of sleep apnea, using any ofthe above techniques.

13. Identify each probable apneic event with an identity marker, oralternatively mark each event by its descending slope or by the sloperatio.

14. Treat the sleep apnea, either automatically, or manually, based on adiagnosis.

15. Repeat steps 1-14 to confirm the diagnosis and efficacy oftreatment.

The above system represents the general concepts of one embodiment ofthe present invention. Other comparisons which incorporate thedesaturation slope and the resaturation slope are also included withinthis teaching.

For example FIG. 5 shows how a comparison can use the calculation of thearea above each desaturation event compared to area above each coupledresaturation event. With this system, an x-axis is projected from apoint of initial desaturation. A second y-axis is projected upward fromthe initial point of rise of saturation which signifies the onset of aresaturation event. The areas above the sloping lines, defined as D andR in the above figure, are then compared in a similar manner to thatdescribed in the previous embodiment.

In addition, the specificity and sensitivity of oximetry with respect tothe diagnosis of sleep apnea is greatly enhanced by another embodimentof the invention which includes all of the multiple slope comparisons asdescribed above. In such a system, in combination, the desaturationslope is compared to a desaturation slope which is consistent with adiagnosis of sleep apnea; second, the resaturation slope is comparedwith resaturation slopes known to be consistent with sleep apnea; third,desaturation slopes are compared with coupled resaturation slopes todefine a slope index which is known to be consistent with sleep apnea;fourth, desaturation slopes and resaturation slopes are compared withother such slopes within the same record to identify slope similarity ofthe desaturation slopes and slope similarity of the resaturation slopes,respectively; furthermore, the similarity of thedesaturation-resaturation slope index of the identified events can becompared; furthermore, as previously discussed; consecutive slopes canbe compared in relationship to the interval between desaturation eventsto determine whether a preceding desaturation event affects the slope ofa closely after following desaturation event, and; finally, the mean ofall desaturation slopes can be compared to the mean of all resaturationslopes to define an aggregate index.

In another embodiment, the present invention identifies a phasicdesaturation event to trigger storage or collection of at least oneadditional parameter of the patient. These additional parameters can be,for example, a recording of sound or video. When the microprocessoridentifies specific coupled desaturation-resaturation parameters whichare physiologically consistent with sleep apnea, the microprocessorinitiates the storage of selected data collected by at least oneadditional sensor.

Sound has been shown to be an important indicator of airway obstruction,however, many patients spend the majority of their night without majorobstructive apneas. Therefore, if the entire night of sound wererecorded, it would include a large amount of unnecessary soundrecording, for only a small amount of useful data surroundingobstructive apneas. In the preferred embodiment shown in FIG. 1, theadditional sensor includes a microphone 30 which can be integral with orcarried by the probe 13 of the pulse oximeter 12 or which can bepositioned in other regions near the patient during sleep. With thispreferred embodiment, the microphone 30 is connected to an audioprocessor 32 of any known type, such as a Sound Blaster(TM) 16-bitprocessor. The sound is recorded digitally as a function of time.Alternately, the sound may be Fast Fourier transformed ("FFT"), and thetransform information may be stored. Alternatively, other means of soundor other recording can be utilized.

Preferably, the sound is continuously recorded throughout the night andthe most recent recording always maintained in short-term memory. If,after a finite period of time (for example, 4 minutes), no coupleddesaturation-resaturation event occurs which is specific for sleepapnea, the oldest part of the recorded sound will be erased or otherwisenot marked for retrieval. If, however, a coupleddesaturation-resaturation event occurs which is consistent with sleepapnea, the identification of this event will trigger the marking andstorage of the collected sound data during an interval preceding,during, and immediately after the event.

In the preferred embodiment, the total sound interval retained for eachdesaturation event includes the interval of the coupleddesaturation-resaturation event, as well as one minute preceding and oneminute following each such event; although this recording time can befurther reduced for greater efficiency of memory utilization. In thisway, the entire night will be monitored by oxygen saturation while soundis stored, but the .information can be rejected to save memory unless asleep apnea event is identified by pulse oximetry. If a sleep apneaevent is identified, this will trigger the long-term storage of soundinformation surrounding that event. In this way, the efficiency samplingof sound that can be greatly enhanced since only small portions of soundneed be collected in relationship to each apnea event.

Continuous recording of oxygen saturation and sound when indicated as afunction of time can be digitally .stored on any commercially availableremovable computer memory media, for example, a high-capacity floppydisc, or a removable Bernoulli disc, and then transported to a secondmicroprocessor for evaluation by the physician and for printing. Theentire record can be printed with a continuous graphical representationof oxygen saturation as a function of time. The sound can be graphicallyrepresented as a function of time by (for example, showing the volume asthe width of the line and the frequency as its position along they-axis). As discussed previously, such graphical representation ofoxygen saturation can include specific markers indicating coupleddesaturation and resaturation events which are physiologicallyconsistent with sleep apnea.

Preferably, staccato or interrupted low frequency sounds may also begraphically represented preceding an oxygen desaturation event.Subsequently, variable high frequency sounds of low volume may beidentified immediately preceding the recovery of oxygen saturation,indicating the presence of post-apnea hyperventilation. The physiciancan easily, therefore, determine whether these oxygen desaturationevents are due to obstructive sleep apnea by identifying the soundparameters with which these coupled desaturation-resaturation events aretemporally associated. Of course, all coupled desaturation events mightnot necessarily be associated with a typical sound pattern. However,throughout the night recording, patients with obstructive sleep apneawould be expected to have typical snoring sounds; whereas, patients withcentral sleep apnea from a periodic breathing or alterations inventilation-perfusion mismatch would not be expected to demonstrate suchsound parameters in relationship to such coupleddesaturation-resaturation events.

The system is further advantageous in that it allows the physician toefficiently focus on portions of the night which are of the greatestinterest. For example, the physician can specify a desaturation eventidentified by the microprocessor as an apnea, then either lookgraphically at the sound surrounding that event or, alternatively,listen to digitally-recorded sound which surrounds a specificdesaturation-resaturation event. It should also be clear that a videorecorder could be activated in a similar manner, along with a soundrecorder, to obtain critical bytes of a night's sleep for efficientevaluation. In this way, the diagnosis of airway obstruction can beconfirmed, along with the diagnosis of sleep apnea, by utilizing agreatly simplified and less expensive system than conventional homepolysomnography.

It is clear that, because of overlap with other disorders, the diagnosisof mild sleep apnea cannot be achieved by identifying a single coupleddesaturation-resaturation event even when the event and all theassociated slopes are physiologically consistent with sleep apnea. Forthis reason, the identification of a desaturation slope and aresaturation slope and a comparison of these slopes, even wherein allmeet the criteria for sleep apnea, can only be said to identify an eventthat is physiologically consistent with apnea from the perspective ofoxygen desaturation and resaturation waveform. It is the comparison ofmultiple desaturation events which is specific for sleep apnea as in thepresent invention.

Although, as per the previous embodiment, the analysis of slopeparameters when multiple events are identified and counted is specificwith respect to moderate to severe apnea, it is critical to achievespecificity for the large patient population that has only mild sleepapnea. Unfortunately, many disorders can produce oximetry waveformdeflections which are repetitive and/or cyclical and of equivalentmagnitude to those of mild sleep apnea.

Enhanced sensitivity must be achieved for patents with mild oximetrydeflections due to sleep apnea. In addition to providing enhancedsensitivity it is important for a system to make a rapid diagnosis ofthe presence of instant sleep apnea for CPAP titration. Themicroprocessor must make a definitive and reliable assessment of thepresence or absence of sleep apnea within a short interval to allow ahigher number of upward CPAP titrations throughout the night to assurethat the minimum opening therapeutic pressure has been identified aswill be discussed.

One preferred embodiment utilizes the continuous calculation andcomparison of saturation slopes to identify sleep apnea to therebyenhance sensitivity for mild apnea and achieve rapid diagnosis ofinstant sleep apnea. In this embodiment, as is conventional, oxygensaturation is measured as a function of time and each saturation datapoint is stored as a function of the sampling frequency. The presentinvention then utilizes each new data point with a preset number ofpreceding data points (for example, four data points wherein thesampling frequency is 20 Hz) to derive a continuous instantaneous slope.The instantaneous slope is recorded as a function of time and can beplotted with saturation as a function of time on the same graph. In thispreferred embodiment, the instantaneous slope is calculated as the slopeof the line of best fit (as by conventional formulas) drawn through thespecified number of saturation points, such as 3-5 data points. As eachdata point is added, the new slope is recorded as a function of this newdata point with the first data point of the group deleted. This derivesa continuous moving waveform of the calculated slope of oxygensaturation/second, which is shown graphically in FIG. 6. In thepreferred embodiment, multiple consecutive slopes in the same directionare considered aggregate slopes and are averaged to produce a meannegative or positive aggregate slope. The continuous calculation andanalysis of this slope waveform provides an enhanced specificity in thediagnosis of sleep apnea with minimal compromise in sensitivity since itis not dependent on a specific threshold deflection for theidentification of apnea.

Since sampling frequency will determine the configuration of anyoximetry waveform. The greater the sampling frequency, the more reliablewill be the slopes in the presence of very mild sleep apnea. For mildsleep apnea, a sample recording interval of 3 seconds (wherein thelowest recorded saturation with this interval is recorded) is adequate,although a continuous sampling for each pulse is optimal for thisdiagnostic system.

In sleep apnea, oxygen desaturations generally occur within clusters.For the purpose of the present invention, a cluster is said to bepresent when at least three consecutive negative slopes interrupted bypositive slopes have occurred wherein the intervening interval betweeneach consecutive negative slope is less than two minutes. The presentinventor has discovered that the presence of a cluster of at least threenegative slopes meeting these criteria and wherein the consecutivenegative slopes are similar (for example, falling within a range of theinitial slope ±60%) and wherein the negative-positive slope ratios arewithin 3.5-10.5 is clearly diagnostic of a sleep apnea cluster and canbe said to comprise a sleep apnea slope cluster complex, referred tohereinafter as a "slope cluster complex." Such a slope cluster complex50 is graphically shown in FIG. 6.

In the presently preferred embodiment, the identification of slopecluster complexes is used to facilitate CPAP titration. With thissystem, the microprocessor can initiate nasal positive pressure at, forexample, a pressure of 4 cm of H₂ O upon identification of a slopecomplex. As is known in the art, this pressure can be incremented froman initial 0 pressure up to 4 cm of H₂ O pressure over a period of twoto five minutes or longer to minimize the potential for arousal withinitiation of therapy. Throughout this time, the pulse oximetry waveformis monitored for any evidence of further slope cluster complexes. If anadditional slope cluster complex occurs after the CPAP has reached 4 cmof pressure, the microprocessor again increases the CPAP level by anadditional 1 cm during the final negative slope of this new complex. Ifan additional after-following slope cluster complex again occurs themicroprocessor again increments, the nasal CPAP pressure by anadditional 1 cm during the final negative slope of this complex. Themicroprocessor will continue to monitor for further complexes andsimilarly, increment the nasal CPAP by 1 cm upon each recurrence up to apresent pressure limit of, for example, 15 cm. When no further suchcomplexes occur subsequent to an increment in CPAP, this level ismaintained for a sustained period, which should preferably be equal toor exceed 15 minutes. If any further slope cluster complexes occurwithin this interval, the microprocessor will increment CPAP by 1 cm ofH₂ O pressure and this pressure will be maintained until no furthercomplexes are identified for 15 minutes. Once the baseline oxygensaturation has been without further slope cluster complexes for 15minutes, the CPAP is eliminated by the microprocessor. This can occurslowly over a period of, for example 2 minutes, to minimize thepotential for arousal to be induced by sudden reduction of nasal CPAP.The patient is then monitored again for evidence of recurrent slopecluster complexes, as previously described. If a slope cluster complexis again identified, the CPAP is incremented in a similar fashion tothat previously described; however, to allow more rapid titration, thestarting level of CPAP is set at a minimum of 2 cm H₂ O below the finaltherapeutic level, which level was achieved during the precedingtitration. For example, if the preceding titration achieved atherapeutic CPAP of 10, the starting titration level for the titrationwould not be less than 8. (However, the CPAP unit can be ramped slowlyup to 8 over a period of 30 seconds, rather than suddenly initiatingthis pressure.) Again, incremental CPAP titration is utilized for eachconsecutive slope cluster complex, as for the initial titration, untilno further slope cluster complexes occur for the specified time intervalof 15 minutes. After the interval of 15 minutes without a slope clustercomplex has concluded, the CPAP will again be withdrawn, as previouslydescribed. The patient will be monitored and, if another slope clustercomplex occurs, a new titration will be initiated. In this way, as manyas 12 or more separate complete CPAP titrations can occur throughout thenight. Actually, however, less CPAP titrations generally will occur in amajority of patients since often there are no more than 4-5 separateclusters of desaturation events in any single night. To increase thenumber of titrations, CPAP may be withdrawn after a shorter specifiedinterval of absent slope cluster complexes, such as five minutes orupward titration may be more rapid, for example with each consecutivenegative slope within a slope cluster complex after an initial 3negative slopes have occurred. In this way, three or four CPAPtitrations may occur within a single 30 minute desaturation cluster.

The comparison of consecutive slopes within a cluster allows increasedspecificity with less loss of sensitivity by accepting the diagnosis ofsleep apnea without requiring a specific magnitude of desaturation fromthe baseline. This is particularly true when the slope cluster complexesare obliterated by initiation or incrementation of the CPAP levels.There is, of course, a time delay between the development of apnea andthe onset of oxygen desaturation identified by the pulse oximeter. Dueto this delay, it is not generally possible to arrest a specificnegative slope by the initiation or upward titration of nasal CPAPduring said negative slope unless the initiation occurs within a veryshort interval after the negative slope has started. Even when initiatedearly, substantial desaturation will continue, even if completeelimination of the obstruction immediately occurs upon initiation. Theinitiation of nasal CPAP during a slope cluster complex, therefore, mayeffectively treat and prevent the next negative slope, but unless theslope is quite prolonged the initiation or upward titration of CPAP maynot interrupt the negative slope which is already in progress since,indeed, the physiological mechanisms causing the negative slope may havealready have been completed. Anticipating this delay (which may be 20seconds or more) the CPAP can be initiated or titrated upwardimmediately upon identification of the third negative slope or at theend of the second negative slope. Arrest of the third negative slopeafter the expected delay can provide diagnostic value.

The purpose of this repetitive cyclic titration is to identify abreakpoint range of CPAP which provides adequate pressure to break acycle of desaturations by preventing further apnea episodes. The effectis diagnostic and further identifies the level of CPAP required forlong-term therapy. The presence of even very small desaturations, whichoccur with slope cluster complexes and which are consistently eliminatedby a finite range of nasal CPAP pressures, is clearly diagnostic ofsleep apnea and specifies the level of CPAP that is required foreffective therapy. The recording of continuous CPAP pressure (as isknown in the art), may be simultaneously performed and, the recurrenttitration of the breakpoint can be identified by plotting the slopewaveform simultaneously with CPAP to verify that actual breakpoints areoccurring as a function of CPAP titration, rather than by chance. It isclear that with any single episode of titration, spontaneous cessationof sleep apnea cycles may occur at any time during the titration,providing an initial "breakpoint" which may actually not be truly afunction of adequate therapy. However, the consistent identification ofa single breakpoint range (for example, 8-10 cm) at which point, forexample, four separate slope cluster complexes were terminated andwherein no further slope cluster complexes occurred when this level wasmaintained would clearly identify adequate therapy and would identifythe lowest adequate therapeutic pressure.

Ideally, the entire titration process occurs over two nights. Theinitial titration process involves recurrent initiation and withdrawalof nasal CPAP, as previously described, over cycles of 15-30 minutesthroughout the night. The microprocessor identifies the breakpointpressure which is adequate to break all slope cluster complexesthroughout the entire night's study. This pressure level is designated"the therapeutic breakpoint CPAP" level and is recorded and stored forthe second night's study without requiring a second home visit or modemcontrol for adjustment of the CPAP. Upon initiation of the secondnight's study the microprocessor automatically ramps the CPAP unit up tothe therapeutic breakpoint CPAP value over a specified interval of, forexample 5-30 minutes. The patient is then maintained on this pressurelevel throughout the night to assure the pressure is adequate.

Importantly, for the sleep apnea diagnostic system to be utilized inclinical medicine, a hard copy must be produced so that the physiciancan overread the interpretation of the computer because many cardiac andpulmonary disorders, as well as artifact, can produce deflections withsimilar magnitude and configuration with that of mild sleep apnea. It iswell known that differentiation of mild sleep apnea by visualizing thepatterns of conventional oximetry waveform plots of oxygen saturationversus time is non-specific. The present invention describes a systemwhich derives and analyzes continuously the multiple slope patterns tomake a definitive diagnosis of sleep apnea quickly, however, for this tobe accepted by the medical community, the physician must have a new wayto interpret the oximetry tracing waveform and thereby overread theinterpretation of the computer. The present inventor has discovered thatthe slope cluster complexes are easily visualized even in the presenceof only mild apnea by graphically representing the continuous,instantaneous slope of oxygen saturation [as Δ Saturation (%)/Δ Time(seconds)] as a function of time, where slope is placed on the y-axisand time is placed on the x-axis. Utilizing this representation, theeffect of the limited magnitude of the deflection is greatly minimized,and the effect of the particular slope characteristics are maximizedgraphically and visually. The effect of visually and graphicallyrepresenting continuous slope as a function of time is shown in FIG. 6.This graphical representation allows the physician to overread theinterpretation of the computer by identifying visually the presence ofslope cluster complexes. As noted, this graph demonstrates the slope ofthe oxygen saturation as a function of time where:

slope=change in saturation ΔS (%)/ΔT (seconds), and where time is inminutes.

The y-axis includes marked regions which identify slopes that arephysiologically consistent with sleep apnea. For example, with respectto negative slopes, the physiologically consistent region is marked as-0.3 to -1.1 and with respect to positive slopes, the physiologicallyconsistent region is marked as 2.5 to 8.3. As oxygen saturation datapoints are measured and stored, continuous slope calculations are made.Alternatively, consecutive negative or positive slopes may be defined asa single aggregate positive or negative slope and may be average for thepurposes of graphical representation and interpretation.

The previously-described apparatus is both a diagnostic tool for sleepapnea and a fixed therapeutic pressure identifier. Specifically, itidentifies the minimum fixed therapeutic pressure than can reliablyprevent substantially all future apnea episodes in a given patient. Thispressure is printed and identified as the minimum therapeutic breakpressure or optimal nasal CPAP pressure. In the preferred embodiment,the microprocessor may set the nasal CPAP pressure on the nasal CPAPunit for long term therapy so that this pressure is subsequentlymaintained for this patient without further adjustment by patient,physician, or home health personnel. This therapeutic pressure which hadbeen previously identified is, therefore, fixed and will be utilized,for example over the next 6-12 months, until a repeat study is performedat this pressure to confirm that further apneas have not redeveloped orthat a lower pressure might be therapeutic (such as after loss).

While this language herein refers to oxygen saturation, it should beunderstood that gas exchange parameters could be determined in waysother than those specifically disclosed herein, but are included withinthe scope of this teaching. For example, sequential and cyclictime-dependent storage of carbon dioxide in body compartments duringsleep apnea can be similarly used to diagnose sleep apnea using, forexample, the comparison of consecutive slopes of maximum exhaled pCO₂.Also, inspiration-triggered variable pressures, such as BIPAP, may alsobe titrated in a similar manner to that described herein for CPAP.

Many modifications will become evident to those skilled in the an fromthis teaching and these modifications are included within the scope ofthis teaching.

What is claimed is:
 1. A sleep apnea treatment device, comprising:anoximeter, having an output signal indicating an oxygen saturation of apatient's blood; a processor element, responsive to said signalindicating oxygen saturation, for identifying at least two consecutivedesaturation events, in which said oxygen saturation falls below abaseline level by a predetermined amount and for a predetermined time,said processor element identifies each desaturation event only when italso includes a resaturation, immediately following said desaturationand coupled with each said desaturation, in which the oxygen saturationrises, said processor element for calculating a slope of each saiddesaturation event representing a rate of change per unit time of fallof oxygen saturation, said processor element for comparing saidcalculated slopes with value of slopes which are determined in advanceto be indicative of sleep apnea and making a diagnosis of sleep apneabased on said comparing and wherein said comparing is also based on atleast one slope of said resaturation; and a positive pressure generatingdevice, constructed and arranged to administer positive airway pressurethrough a patient's nasal pharynx, and wherein said processing elementactuates said positive pressure activating device upon identification ofsaid at least two consecutive slopes, said positive pressure generatingdevice being activated by oxygen saturation of the patient to therebymitigate further desaturation.
 2. An apparatus as in claim 1 whereinsaid processing element compares said consecutive slopes with anabsolute number which is likely to indicate sleep apnea.
 3. An apparatusas in claim 1 wherein said processing element compares said consecutiveslopes with other slopes taken at different times.
 4. An apparatus as inclaim 1 wherein said processing element includes means for calculating aratio of resaturation slope to desaturation slope and comparing saidratio with a predetermined number to diagnose sleep apnea.
 5. Anapparatus for treating sleep apnea comprising:an oximetry sensor of atype which determines an oxygen saturation of a patient's blood; amicroprocessor, programmed to recognize outputs from said sensor and tocalculate an oxygen saturation of a patient's blood as a function oftime from said outputs, and to recognize a coupleddesaturation/resaturation occurrence indicative of sleep apnea, saidoccurrence including:a. a desaturation event when said oxygen saturationfalls a predetermined amount in a predetermined time, and b. aresaturation event when said oxygen saturation rises a predeterminedamount in a predetermined time within a predetermined time after saiddesaturation event, said microprocessor also including means forcomparing a slope of desaturation of said desaturation event, with aslope of resaturation of said resaturation event, and using saidcomparison to diagnose sleep apnea, a positive pressure generator foradministering positive airway pressure through the patient's nasalpharynx; and said microprocessor including means for activating saidpositive pressure generator upon identification of said occurrence ofsleep apnea so that upon identification of said occurrence, saidpositive pressure generator is activated to the patient to therebymitigate further desaturation.
 6. A device for diagnosing sleep apnea,comprising:an oximeter, having an output signal indicating an oxygensaturation of a patient's blood; a processor, operating to:1) identify afirst desaturation event, in which said oxygen saturation falls below abaseline level by a predetermined amount for a predetermined time, 2)identify a second desaturation event, in which said oxygen saturationfalls below a baseline level by a predetermined amount and for apredetermined time; and 3) trigger a storage of said continuouslyrecorded information for a predetermined time responsive to saididentifying said first and second desaturation events, and otherwisecommanding that said continuously recorded information not be stored. 7.A method for identification of sleep apnea and for determining anoptimal level of nasal positive pressure for the treatment of saididentified apnea, the method comprising the steps of:a. disposing anoximeter adjacent to a single human body part and recording continuousmeasurement of oxygen saturation as a function of time to derive anoxygen saturation waveform; b. defining a time interval for calculationof a slope of the oxygen saturation; c. using a computer to continuouslycalculate said slope along said waveform, using said time interval; d.continuing executing said steps b and c and using the computer toidentify at least two consecutive slopes which fall within a specificrange; e. upon said identification in said step d), using the computerto initiate a preset positive pressure to the nasal pharynx with apositive pressure device; and f. subsequent to said initiation of saidnasal positive pressure, continuing to calculate said slopes of saidoximetry waveform to determine efficacy of said nasal positive pressurein mitigating further presence of sleep apnea as defined by the absenceof said consecutive slopes.
 8. The method of claim 7 further includingthe step of comparing at least the first and second slope of saidconsecutive slopes and initiating said nasal positive pressure based onsaid comparing.
 9. The method of claim 7 further including the step ofidentifying at least a first and a second consecutive desaturation eventin which the oxygen saturation falls below a baseline level for apredetermined amount and for a predetermined time, and wherein saidcalculated slopes are calculated during said events.